1. Field of the Invention.
The invention relates to laser gyroscope systems and particularly to such systems employing waves of four different frequencies within the laser gyro cavity.
2. Description of the Prior Art.
In general, laser gyroscopes devices have two or more waves traveling in opposite directions along a closed path including a laser gain medium so that rotation of the device about an axis within the path causes the path length for oppositely rotating waves to differ depending upon the rate of rotation. With a two wave or frequency system, it has been found that, for low rates of rotation corresponding to a small theoretical difference frequency, the actual output difference frequency is zero or substantially less than would be expected due to the phenomena known as lock-in. It is believed that the lock-in problem arises because of coupling between the waves which may arise from a number of possible factors including back scattering of laser energy from elements within the laser path such as mirrors or a polarization dispersive structure or from scattering centers within the laser gain medium itself.
The earliest attempts to compensate for this problem included one proposal in which the two beams are biased at zero rotation away from the zero output level by the use of a Faraday rotator which subjects beams propagating in different directions to different delay times. Unfortunately simply biasing the two beams sufficiently far apart to avoid lock-in produced a large frequency difference between the two beams, so large in fact that the change in frequency caused by ordinary amounts of rotation was rather insignificant compared to the total frequency difference. Thus, any small drift could obliterate the actual desired signal output. Further attempts to achieve biasing included one in which the Faraday rotator was switched from one direction to another using a symmetric AC switching waveform. Such systems have proven somewhat difficult to implement since the symmetry of the AC switching waveform had to be maintained to greater than one part in a million.
The most successful laser gyroscopes yet proposed and constructed employ four waves of two pairs or beams each propagating in opposite directions. Such systems are shown and described in U.S. Pat. Nos. 3,741,657 and 3,854,819 to Keimpe Andringa and assigned to the present assignee, the specifications of those patents being herein incorporated by reference. In such a laser system, circular polarization for the four waves is preferred. The pair of waves propagating in the clockwise direction includes both left and right hand circularly polarized waves as does the pair propagating in the counter clockwise direction.
Two biasing components are provided. A device such as a crystal rotator produces a delay for circularly polarized waves that is different in one sense or handedness of circular polarization than for the opposite sense and is also reciprocal. That is, a wave traveling in either direction through the crystal will be delayed by the same amount of time. Secondly, a device such as a Faraday rotator is also disposed in the wave path. Such a device is nonreciprocal providing a different time delay for the two directions of propagation. This is achieved by rotating the circular polarization vector by different angles. The delay is independent of the sense of polarization. The result of these biasing operations produces four waves, two with frequencies above the peak of the gain curve of the laser medium and two below. The two above may for example both be right-hand circularly polarized while the lower two are left-hand circularly polarized. At a zero rate of rotation, the frequency difference between the left-hand circularly polarized and the right-hand circularly polarized waves are equal. When, for example, the system is rotated in one direction the right-hand circularly polarized waves will move closer together in frequency while the left-hand circularly polarized waves will move apart. The opposite direction of rotation produces the opposite direction of change in frequencies. The actual rotation rate is readily related to the difference between the difference in right-hand circularly and left-hand circularly polarized wave pairs.
In the laser gyroscope systems disclosed in the referenced patents, a structure for adjusting the length of the path through which the four waves propagate to maintain the frequency pairs positioned symmetrically about the center maximum gain frequency of the laser gain medium curve is described. Such symmetric positioning is desired in order to minimize residual drift or lock-in effects.
It has been desired to provide output processing circuitry for determining both the rate or amount of rotation and for providing path length control. Previously known output processing circuit required matched gain amplifiers for providing path length control. The gains of two amplifiers had to be maintained within tight tolerance limits of each other over a wide range of temperature and environmental conditions thereby making such circuits both costly and complex.
In other known prior art sytems a great deal of power was wasted or used inefficiently in the output processing circuitry.
Accordingly, it is an object of the present invention to provide a laser gyroscope system having efficient and economical processing of output signal data.
It is also an object of the present invention to provide such a laser gyroscope system in which path length control is achieved without the use of matched gain amplifiers.
Furthermore, it is an object of the invention to provide a laser gyroscope system which requires a minimum amount of power to be extracted from the wave path for operation of the output and path control circuitry.